1. Field of the Invention
The present invention relates generally to a multi-antenna multi-user communication system, and in particular, to an optimum perturbation apparatus and method.
2. Description of the Related Art
In a wireless communication system where a Base Station (BS) with multiple antennas send data to multiple users simultaneously, channel inversion, regularized inversion, vector perturbation, and regularized perturbation are used for precoding.
The channel inversion scheme, also called zero-forcing precoding, is simplest in that users receive their own data symbols without any coordination among them. The transmitted signal with channel inversion is given by Equation (1),
                    x        =                                            P                                      r                                ⁢                                                    H                H                            ⁡                              (                                  HH                  H                                )                                                    -              1                                ⁢          u                                    (        1        )            where r is a normalization factor expressed as ∥HH(HHH)−1u∥2, P is a transmitting power and u is a data vector. r can be increased to infinity due to inversion of a channel response matrix in a poor channel environment. As a consequence, Signal-to-Interference and Noise Ratio (SINR) may become very small.
To overcome the problem of the SINR decrease, the regularized inversion scheme uses HH(HHH+αI)−1 instead of HH(HHH)−1 in Equation (1). Thus, the transmitted signal is changed to Equation (2),
                    x        =                                            P                                      r                                ⁢                                                    H                H                            ⁡                              (                                                      HH                    H                                    +                                      α                    ⁢                                                                                  ⁢                    I                                                  )                                                    -              1                                ⁢          u                                    (        2        )            where r=∥HH(HHH+αI)−1u∥2, I is a unitary matrix. SINR can be maximized by selecting an optimum value of α with which r is bounded.
In the vector perturbation scheme, a data vector u is perturbed by an integer-offset vector. Compared to the regularized inversion scheme that maximizes SINR without using a modulo operation at a receiver, the vector perturbation scheme finds an integer vector maximizing SINR and shifts the constellation as much as the integer vector under the assumption of the modulo operation at the receiver. The transmitter sends a signal represented by Equation (3)
                    x        =                                            P                                      r                                ⁢                                                    H                H                            ⁡                              (                                  HH                  H                                )                                                    -              1                                ⁢                      (                          u              +                              τ                ⁢                                                                  ⁢                1                                      )                                              (        3        )            where r=∥HH(HHH)−1(u+τl)∥2, τ is a positive real number, and l is a Kx1 complex vector. The receiver recovers the received signal by eliminating τl by a modulo operation. To maximize the SINR of the received signal, the BS chooses l to minimize r as defined by Equation (4),
                    1        =                              argmin                          1              ′                                ⁢                                                                                                                              H                      H                                        ⁡                                          (                                              HH                        H                                            )                                                                            -                    1                                                  ⁢                                  (                                      u                    +                                          τ                      ⁢                                                                                          ⁢                                              1                        ′                                                                              )                                                                    2                                              (        4        )            This is an integer-lattice least-square problem, which is solved by a sphere encoder.
Finally, the regularized perturbation scheme simply combines the regularized inversion scheme with the vector perturbation scheme. The transmitted signal is defined by Equation (5),
                    x        =                                            P                        r                    ⁢                                                    H                H                            ⁡                              (                                                      HH                    H                                    +                                      α                    ⁢                                                                                  ⁢                    I                                                  )                                                    -              1                                ⁢                      (                          u              +                              τ                ⁢                                                                  ⁢                1                                      )                                              (        5        )            where r=∥HH(HHH+αI)−1(u+τl)∥2. As with the vector perturbation scheme, to maximize the SINR of the received signal, l is chosen to minimize r as reflected by Equation (6) as follows.
                    1        =                              argmin                          1              ′                                ⁢                                                                                                                              H                      H                                        ⁡                                          (                                                                        HH                          H                                                +                                                  α                          ⁢                                                                                                          ⁢                          I                                                                    )                                                                            -                    1                                                  ⁢                                  (                                      u                    +                                          τ                      ⁢                                                                                          ⁢                                              1                        ′                                                                              )                                                                    2                                              (        6        )            
As the vector perturbation scheme is additionally used after the regularized inversion scheme, the regularized perturbation scheme neither maximizes SINR and nor optimizes the parameter α for the regularized inversion scheme. Therefore, interference still remains in the receiver signal after the modulo operation.